1. Technical Field
The present invention relates generally to radio devices and in particular a method and system for radio frequency (RF) group delay compensation in a broadband orthogonal frequency division multiplexing (OFDM) system.
2. Description of the Related Art
A constant group delay from an RF filter generally has little to no impact on a receiver's performance. Therefore, when designing a RF filter, the group delay variation should be kept as small as possible within a signal bandwidth. For broadband OFDM systems, the transmitted signal is very susceptible to a phase offset due to a group delay.
In operation, the group delay of a RF filter or combiner varies based on variations in ambient or environment temperatures. Thus, as the ambient temperature changes, the group delay within a signal bandwidth consequently varies. Therefore, maintaining a constant group delay in a particular RF filter or combiner of a particular receiver is virtually impossible.
For example, FIG. 8 shows a plot of two group delay measurements P1 and P2 (in ns) versus frequency. The group delay measurements are for a RF cavity combiner used in a Worldwide Interoperability for Microwave Access (WiMAX) base station in ambient temperatures 50° C. (P1) and −40° C. (P2), respectively. As can be readily seen, those tones close to a band edge suffer a large group delay variation. The center of the band corresponds to a frequency of 0 Hz. In the center of the band, the group delay is more constant as compared to those frequencies moving in the direction of the band edge.
The receiver's equalizer could compensate for some phase offset due to a group delay. In the case of a low level modulation and coding scheme (MCS), such as QPSK modulation, the system performance degradation would be negligible for group delay compensations carried out by the receiver's equalizer. However, when the MCS level is high (e.g. 64 Quadrature Amplitude Modulation (64 QAM)) and a large group delay variation close to the band edges is present, the group delay will significantly impact the receiver's performance. For example, the receiver's performance may result in a failure associated with a sensitivity test at BER 10−6 in WiMAX radio conformance tests (RCT).
The large performance degradation of high MCS resulting from group delay is contributable to an uncorrectable portion of a phase error associated with a linear equalizer that is implemented in an OFDM receiver. Measurements indicate that the uncorrectable phase error within a slot associated with a linear equalizer, such as for 2×3 adaptive modulation and coding (AMC), could be a number of degrees. Thus, known linear equalizers in OFDM receivers do not compensate for a large RF filter group delay variation.